In measurement of a true density using a gas phase substitution method, a sample chamber in which an object to be measured is housed is filled with an inert gas, such as He gas, and the inert gas is subsequently released into an expansion chamber, to find the volume of the object to be measured based on a change in an internal pressure of the sample chamber resulting from the release, and convert the found volume of the object to be measured into a true density. A concrete method for measurement is specified in Japanese Industrial Standard (JIS) Z8807.
As a true density measurement device based on the gas phase substitution method, for example, a device disclosed in Patent Document 1 has been known. The device of Patent Document 1 employs no arrangement for allowing the capacity of an expansion chamber to be changed.